Production of acetic acid



March 1958 G. P. ARMSTRONG ET AL 2,325,740

PRODUCTION OF ACETIC ACID Filed 001:. 1, 1954 @L 11 13 1o 5 i, N f

INVENTORS.

Ufli Sta es ,P ten fi 2,825,740 Patented Mar. 4, 195.8

2,825,740 PRODUCTIUN F ACETIC ACID Godfrey Paul Armstrong, Kingswood, and Alfred Frank Millidge, Coulsdon, Englandassignors to The Distillers Company Limited, Edinburgh, Scotland, :1 British company The present invention relates to improvements in a process for the production of acetic acid by the liquid phase oxidation with molecular oxygen of paraflinic hydrocarbons containing 4-8 carbon atoms.

It is an object of the invention to provide a process in which 4-8 carbon atom parafiinic hydrocarbons are oxidised under specified conditions so that optimum yields of acetic acid are obtained and the mixture of aliphatic acids produced contains a high proportion of acetic acid.

According to the present invention the process for the production of acetic acid comprises oxidising parafiin hydrocarbons of 4-8 carbon atoms in the liquid phase at a temperature in the range 150-220 C. with molecular oxygen continuously withdrawing part of the reaction mixture and passing it to a distillation zone, distilling said part to remove as distillate the material boiling in the presence of water up to 99 C., and returning this distillate with the fresh hydrocarbon feed to the oxidation zone, the throughput rates of the oxygen and the hydrocarbons being such that the ratio, hereinafter termed A, of the weight of oxygen taken up per hour to the weight of the fresh hydrocarbon fed plus the weight of the distillate returned to the oxidation zone per hour is at least 0.5 and recovering acetic acid from the residues of the distillation.

The ratio A varies with the residence time per pass and/or the oxygen uptake rate, shorter residence times (i. e. faster total feed rates to the oxidation zone) or lower oxygen uptake rates leading to lower values for A. In practice, A can be controlled from a figure approaching zero to a figure of about 1.0, simply by adjustments of the total feed rate to t.e oxidation zone, and of the air (or oxygen) rate through the oxidation zone.

The withdrawal of the reaction mixture from the oxidation zone may be carried out in any suitable way. it has been found that the reaction mixture is normally a homogeneous liquid at operating temperature, but that on cooling two liquid layers separate. Of these the upper layer is composed mainly of unchanged hydrocarbon, while the lower layer contains the bulk of the acidic products and water. it is preferred to operate by removing continuously or intermittently a part of the reaction mixture as a homogeneous liquid, to cool this to a temperature below about 80 C. and preferably to as near room temperature as may be convenient, to separate the liquid layers, to return the upper layer without treatment to the oxidation zone and to pass the lower aqueous layer or a part thereof to the distillation zone, which may consist of one or more distillation stages. According to a preferred embodiment of the invention the ratio, hereinafter termed B, of the weight of the distillate returned to the oxidation zone per hour to the weight of the lower aqueous layer passed to the distillation zone per hour is not greater than 0.55.

The parafiin hydrocarbon or mixture of hydrocarbons of four to eight carbon atoms employed for the oxidation is desirably one which does not exert too high a mixture is maintained 2 vapour pressure, specifically those which are normally liquid at room temperatures of about 30 C. and atmospheric pressure, such as normal pentane and the C to C parafiins, and mixtures thereof.

The oxidation may be effected with any gas containing molecular oxygen, Whether in the form of air or of mixtures poorer or richer in oxygen than is air; part of the molecular oxygen may, if desired, be in the form of ozone.

The use of superatmospheric pressure in the oxidation will generally be necessary in order to maintain a major part of the reactants in the liquid phase.

The temperature of oxidation should be sufiiciently high to provide an economically attractive rate of oxidation and a low level of peroxidic compounds. Thus, the temperature should not be so low that insufiicient oxidation to acids occurs or alternatively be so high that excessive oxidation occurs of the hydrocarbons, or of their primary oxidation products, to oxides of carbon and water. Temperatures in the range about C. to 200 C. are preferred, although it is also possible to use higher or lower temperatures.

The oxidation may be carried out in a variety of types of apparatus, provided only that the reaction substantially in the liquid phase, and that adequate contacting is effected between the liquid reaction mixture and the oxidising gas. Thus for example, the oxidation may be carried out in a reactor in the form of a vertical tower with the feed of oxidising gas at the base, or at a number of points up the tower, in which case the necessary agitation is efiected by the gas itself; this case it is advantageous to subdivide the gas feed by mechanical means to obtain a fine dispersion at the point or points of entry and throughout the reaction zone. Alternatively, the oxidising gas may be fed into a stream of liquid moving with high velocity in a circulatory system, such as a coil reactor with forced circulation, whereby turbulent conditions of flow are obtained. Other types of apparatus and means for efi'ecting the agitation intermixing of the reactants may also be used.

These oxidations may be carried out in the absence of any added catalyst, or in the presence of a suitable oxidation catalyst. Such catalysts include compounds of those metals that are capable of existing in more than one valency state. Examples of suitable metals include manganese, cobalt, nickel, vanadium and copper. These catalysts may be conveniently added in the form of their oil-soluble salts with organic acids, or alternatively the catalyst metal may be added in the form of an anion, whether as the free acid or a salt thereof for example as a vanadate.

The acetic acid may then be recovered by any suitable method, for instance by azeotropic drying of the wet acids followed by fractional distillation.

It is preferred to employ materials for the construction of the oxidation reactor and equipment which possess a sufficient degree of resistance to organic acids, although the corrosive properties of the oxidation mixture are less than those of the most highly corrosive constituents. Thus, it has been found that a stainless steel containing 18% chromium, 8.5% nickel, 0.6% titanium (Firth- Vickers PDP) is a satisfactory material of construction, although certain of the minor constituents of the mixture, such as formic acid, are by themselves corrosive towards this steel.

The following examples are given to illustrate the procass of the present invention.

Example 1 hing, and recycled to the oxidation reactor.

less tube 1 2.6 inches indiameter and 6 feet high, having air feed points '2 at the base and about the mid-point. Fresh hydrocarbon is introduced continuously by line 3 through a preheater 13. Both gaseous and liquid reaction products are removed from the reactor by line 4, and after cooling in cooler 5 are fed to gas/liquid separator 6,'from which the waste gases are withdrawn by line 7. Small amounts of hydrocarbon in the waste gas may be recovered in part by suitable means, for example by oil scrub The cooled liquid products pass to liquid/liquid separator 3, Where the aqueous acid product phase is allowed to settle as a lower layer, part of this layerbeing withdrawn as product through line 9. .The upper hydrocarbon layer is returnedto the reactor through preheater it with the remainder of the separated lower layer. The whole of the withdrawn aqueous acid product is fed by line'9 through a preheater (not shown) to the mid-point of a packed continuous distillation column 11 which has the equivalent of 5 theoretical plates above and below the feed point. The column is operated at atmospheric pressure with a reflux ratio of 4:1, the temperatures being approximately 66- C. at the head, approximately 82 C. at the feed point, and 104-105 C. in the reboiler the distillate passes from the top of the still to a cooler 15 and a separator 16. The total distillate minus reflux is returned by line '12 to the oxidation reactor after admixture with'the fresh hydrocarbon feed. The base product from the still is withdrawn by line 14, and submitted to further distillation processes to recover the acids formed as main product.

The oxidation was carried out in the apparatus described above, using as hydrocarbon feedstock a straightrun gasoline fraction, B. P. -95 from Middle East petroleum. The conditions obtaining during the run were as follows:

The product comprising acids of l4 carbon atoms contained 74.3% acetic acid.

Example 2 An oxidation was carried out in'the same equipment as described in Example 1, using the same feedstock, but under the following conditions; 7 Temperature: 200 C. Pressure: 600 lbs. per .square inch :gauge.

Hydrocarbon feed rate to reactor: 560 grams per hour; Recycled distillatefrom' 11 'to reactor; 470 grams per xhour; V

Total feed rate to reactor: 1030 grams per hour;

Oxygen uptake (absorption) rate: 816 grams per hour;

Product withdrawal rate=feed rate to 11:1 grams per hour.

. These operating conditions gave the following values for A and B.

The product comprising acids of 1-4 carbon atoms contained 77.9% acetic acid.

By way of comparison, an oxidation was carried out in the same equipment as described in Example L'using the same feedstock but under the following conditions:

These operating conditions gave the following values for A and B layer containing unreacted hydrocarbon to the oxidation zone, distilling components of the lower water layer containing'the acetic acid, recovering acetic acid, and returning the portion boiling up to about 99 C. in the presence of water to the oxidation zone, the throughput rates of the oxygen and the reactants being such that the ratio of the weight of the oxygen takenup per hour to the weight of the fresh hydrocarbon fed plus the weight of the distillate returned to the oxidation zone per hour is from about 1 to 0.5. a

2. A process according to claim 1 wherein the ratio of the weight of the distillate returned to the oxidation zone per hour to' the weight of the lower aqueous layer passed to the distillation zone per hour is not greater than 0.55.

References'Cited in the file of patent UNITED STATES PATENTS Feb. 7. i956 

1. A CONTINUOUS PROCESS FOR THE PRODUCTION OF ACETIC ACID WHICH COMPRISES OXIDISING PARAFFINIC HYDROCARBONS OF 4-8 CARBON ATOMS IN THE LIQUID PHASE AT TEMPERATURE OF ABOUT 150-200*C. AND AT SUPERATMOSPHERIC PRESSURE WITH MOLECULAR OXYGEN, CONTINUOUSLY COOLING AND SEPARATING THE LIQUID REACTION MIXTURE INTO TWO LAYERS, RETURNING THE UPPER LAYER CONTAINING UNREACTED HYDROCARBON TO THE OXIDIZATION ZONE, DISTILLING COMPONENTS OF THE LOWER WATER LAYER CONTAINING THE ACETIC ACID, RECOVERING ACETIC ACID, AND RETURNING THE PORTION BOILING UP TO ABOUT 99*C. IN THE PRESENCE OF WATER TO THE OXIDIZATION ZONE, THE THROUGHPUT RATES OF THE OXYGEN AND THE REACTANTS BEING SUCH THAT THE RATIO OF THE WEIGHT OF THE OXYGEN TAKEN UP PER HOUR TO THE WEIGHT OF THE FRESH HYDROCARBON FED PLUS THE WEIGHT OF THE DISTILLATE RETURNED TO THE OXIDATION ZONE PER HOUR IS FROM ABOUT 1 TO 0.5. 